When CMOS processing technology node is reduced to 32 nm and below, a metal gate is usually used in the semiconductor device. Such a semiconductor device also comprises a gate dielectric layer 14 carrying a metal gate 16 and a sidewall spacer 18 arranged on both sides of the metal gate 16 as well as the gate dielectric layer 14, the gate dielectric layer 14 and the sidewall spacer 18 being formed on a semiconductor substrate 10. There are isolation regions (e.g. STI) 24 between the individual semiconductor devices. The semiconductor device also comprises source and drain regions 12 and contact plugs 30 (a metal silicide layer 20 is formed on the surface of the source/drain region 12 prior to forming the contact plugs 30). The source/drain regions 12 are located on both sides of the metal gate 16 and are at least partially buried into the semiconductor substrate 10. The contact plug is embeded into an interlayer dielectric layer 22.
Generally, the contact plug can be a tungsten contact plug or a copper contact plug. The copper contact plug comprises a barrier layer 40 and a copper layer 42 that is arranged adjacent to the barrier layer 40. The barrier layer 40 can be made of Ta and/or TaN. The contact plug 30 connected to the source/drain region 12 via the metal silicide 20 as well as the contact plug 32 connected to the metal gate 16 are shown in FIG. 1 and FIG. 2, respectively.
It is found in practice that using a copper contact can effectively reduce the contact resistance, and thus improve the performance of the devices. At the same time, it might induce the risk of degradation of the device reliability, because it is of great possibility to form a copper silicide when copper contacts the metal silicide 20, which will generate voids in the contact plug or even disconnects the plug. Ta and/or TaN has already been used in the copper contact technique to form the barrier layer 40 for preventing copper from diffusing into the metal silicide 20, i.e. using the barrier layer to separate copper from the metal silicide. However, said layer 40 with a limited thickness can hardly achieve effective separation. The main problem to be solved by the present invention is how to enhance the reliability of the device when using the copper contact technique.